<p><p><div><h2>Project Summary/Scope:</h2>The construction was to replace 300 meters of an existing roadway with a new roadway with wider shoulders. The road is located next to the Snake River. Cut and fill was not an option for two reasons. The first was bank erosion from the river causing a landslide to occur in a 20-meter bank below the roadway. The second was because of a creeping landslide extending 450 meters above the roadway. A retaining wall system would have to be put in place in order to widen the road and to stabilize the landslides. A soil nail wall was chosen to be constructed below the roadway and an SMSE system with a soil nail wall as shoring was constructed above the roadway because of issues pertaining to limited space. This case history is the first known document detailing soil nails being used to stabilize an active landslide.</p><p>Subsurface Conditions: There were three different layers of soil found at this site. The top layer varied between clayey gravel and clayey sand. The middle layer was a medium stiff to stiff, low plasticity sandy clay. Because of lenses of gravel the soil classification was changed to clayey to silty gravel. The bottom layer was very stiff to hard, dark gray shale.</p><p>Soil strength parameters were based on direct shear tests previously run by WYDOT the year before the project. The lower soil nail wall had a height ranging from 1.8 to 3.0 meters with a nail length of 12.2 meters. The spacing of the nails in the lower wall was 1.2 meters vertically and 1.8 meters horizontally. The upper soil nail wall had a height of 4.0 to 7.6 meters with a nail length of 10.0 meters. The spacing of the nails in the upper wall was 1.7 meters horizontally and 2.1 meters vertically. Soil nail pullout capacities were predicted using guidelines from Byrne et al. (1998). The computer program SNAIL was used during the design process as well. Grade 420 steel bars with a diameter of 32 mm were used for the lower wall and a diameter of 25 mm were used for the upper wall. The nail hole diameters were 176 mm. All nails were placed at 15 degrees below horizontal. The MSE wall was constructed on the bench and consisted of modular block masonry facing and galvanized metallic reinforcement with select backfill (φ=33̊, cohesion=33.5 kN/m²).</p><p>During construction the soil nails were installed by inserting the bar into the boreholes. If conditions were encountered where the borehole would collapse, casings were used to reinforce the hole and then removed after the nail was placed. The lower wall was 221 meters long with 308 nails spaced 1.2 meters vertically and 1.8 meters horizontally. The reinforced shotcrete was 215 mm thick. This wall was then buried beneath a structural fill. The upper wall was 226 meters in length and contained 418 nails spaced 1.7 meters horizontally and 2.1 meters vertically. The MSE wall, which extended across the 226 meters of the upper soil nail wall, had a maximum height of 7.6 meters and a facing area of 1,100 square meters.</p><p><figure id='attachment_3597' style='max-width:1028px' class='caption aligncenter'><img class="wp-image-3597 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Schematic showing a typical cross section of a shored wall behind a mechanically stabilized earth wall." width="1028" height="445" /><figcaption class='caption-text'> Typical cross section of soil nail and MSE wall. (Turner and Jensen, 2005; With permission from ASCE)</figcaption></figure><h2>Performance Monitoring:</h2>Strain gages were attached to select nails in both upper and lower walls to measure axial strain. Observations from strain gages placed in the upper wall showed that a face failure had occurred. Three layers of welded wire fabric were used in place of using reinforced shotcrete for a temporary facing. It is recommended to use shotcrete for temporary facing in the future. The MSE wall was still able to be constructed.</p><p>Two inclinometers were installed to monitor ground deformation before, during, and after construction. Overall deformation was found to be within an acceptable range. The project has continued to perform well.<br><h2>Cost Information:</h2>The final contract cost was $2.8 million.<br><h2>Project Technical Paper:<strong> </strong></h2>Turner, J.P. and Jensen, W.G. (2005). “Landslide stabilization using soil nail and mechanically stabilized earth walls: Case study.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 131(2), pp. 141-150.</p><p><em><a href="http://ascelibrary.org/doi/abs/10.1061/%28ASCE%291090-0241%282005%29131…; </em><h2>Date Case History Prepared:</h2>November 2012</p><p></div></p></p>